Tandem trailer arrangements are commonly used with semi-tractor trailer rigs, usually when two or three trailers are towed by a single truck tractor. In operation, a first trailer is connected to the truck tractor using a conventional kingpin and fifth wheel arrangement. A second, tandem trailer is conventionally coupled to the first, lead trailer using a converter dolly carrying a second fifth wheel. The second fifth wheel is pivotally connected to a kingpin mounted on the second trailer, and a hitch at the rear end of the first trailer is pivotally connected to a drawbar eye at the forward end of a drawbar assembly forming the forward portion of the converter dolly. In a three-trailer rig, a second converter dolly couples a third trailer to the second trailer.
A problem with tandem arrangements is that a large gap between adjacent trailers in a multiple trailer rig increases the aerodynamic drag of the rig, which reduces fuel economy. However, if the adjacent trailers are attached too close to each other, they will interfere with each other during turning maneuvers such as those encountered in city driving. In addition, highway regulations require minimum spacing between axles for adequate load distribution on vehicles travelling over bridges. Thus, the distance between trailers must be optimized to reduce aerodynamic drag without violating load distribution regulations or affecting the ability of the rig to maneuver around tight comers.
For a converter dolly having a fixed distance between the drawbar eye and the fifth wheel, two dimensions control the distance between trailers. The first dimension is the distance between the hitch on the lead trailer and the rear end of the lead trailer. The second dimension is the distance between the forward end of the tandem trailer and its kingpin. Because these two dimensions often vary between trailer models, there is a need for a converter dolly in which the distance between the drawbar eye and the fifth wheel can be conveniently established during the manufacture of the dolly and adjusted after the dolly has been assembled to establish the spacing between adjacent trailers for optimized load distribution, maneuverability, and aerodynamics of the trailer rig.
In conventional converter dollies, the fifth wheel assembly and a suspension mechanism supporting a converter dolly axle assembly are welded in position on a frame that is connected to the drawbar assembly at a fixed distance from the drawbar eye. In order to provide a wide variety of length and hitch height configurations, conventional dollies must be manufactured in a wide variety of drawbar and frame configurations. After a conventional converter dolly has been assembled and welded together, its length can be changed only by cutting the fifth wheel assembly from the frame with a cutting torch and re-welding the fifth wheel assembly at a new position on the frame, or by cutting the drawbar assembly from the frame and welding in its place a drawbar assembly of different length. A similar procedure must be followed to change the drawbar eye height. Cutting the dolly and rewelding it is an undesirable adjustment procedure because it is time consuming and weakens the frame.
When changing the position of the fifth wheel, it is also desirable to adjust the position of the axle assembly to maintain its alignment with the fifth wheel so that the converter dolly pivots at the kingpin relative to the tandem trailer. Misalignment of the dolly axle and the kingpin results in undesirable lateral movement of the forward end of the tandem trailer relative to the dolly axle when pivoting during turning maneuvers. Misalignment also places the tandem trailer load forward or aft of the axle and wheels of the dolly and thereby increases the stress on its frame and the wear on the drawbar eye and trailer hitch. Further, if the fifth wheel assembly is improperly located aft of the axle on a conventional dolly, the dolly tracks in an unstable manner as a consequence of compression of the axle suspension, which causes the axle and wheels to wander laterally. To overcome this instability, the fifth wheel on a conventional dolly is typically located one to three inches forward of the axle so that the axle and wheels follow the trailer load and thereby apply tension to the suspension so that the dolly wheels track a stable course.
In a conventional converter dolly, a substantial portion of the frame is occupied by the suspension mechanism that supports the axle assembly, leaving little space available for adjustment of the position of the suspension mechanism along the frame. Thus, the limited range of adjustment of the suspension mechanism effectively restricts the amount of adjustability of the distance between the drawbar eye and the fifth wheel assembly. An additional disadvantage is that the axle assembly and suspension must be cut from the frame with a cutting torch and rewelded in the same manner as the fifth wheel assembly.
One design for an adjustable length converter dolly is described in U.S. Pat. No. 5,338,050 of Haire et al. This converter dolly has a single telescoping drawbar fitted within a sleeve on the frame. The drawbar has a number of holes evenly spaced along its length for drawbar adjustment. Two pneumatic cylinders mounted on the sleeve actuate two pins to couple a pair of holes in the sleeve with a pair of the holes in the drawbar. One disadvantage of this converter dolly is the expense stemming from the use of pneumatic cylinders, switching equipment for controlling the pneumatic cylinders, and equipment for supplying compressed air to the cylinders. In addition, the single drawbar arrangement must be constructed of more massive members than those of a conventional drawbar assembly to overcome an inherent weakness in the drawbar where the pins couple the drawbar to the sleeve. The increased mass of the single drawbar design results in higher manufacturing costs and a heavier rig. This design cannot accommodate different hitch heights without tilting the dolly frame, and requires the drawbar to extend horizontally to prevent the dolly from tilting when changing the drawbar length. Lastly, the single drawbar configuration has less space available for holding air brake equipment, electrical cables, and auxiliary equipment such as a swivel jack for use during storage of the converter dolly.